1. Field of the Invention
This invention relates to automatic focus adjusting devices for use in still cameras, video cameras and the like.
2. Description of the Prior Art
Various range finding devices have been proposed for automatic focus adjusting devices. Auto-focus devices are now even available in the video camera market. As is well known in the art, the range finders employing these facilities may be divided into the so-called active type and passive type. The active type of automatic focus adjustment projects light or sound from the camera onto an object to be photographed and uses the reflection as an information source. In common photographic situations, a subject at a range of distance from a minimum to, for example, 10 meters, produces an effective signal for obtaining the object distance. For an object beyond that range, however, the range finder does not operate reliably because the reflected light or sound level is too low.
In the art of video cameras equipped with the active type automatic focus adjusting device, when the reflection of the projected light or sound is not sensed because the object distance is too long, it has been the common practice to stop the focus-adjusting lens component of the photographic objective at a prescribed position. The maximum object distance at which the reflection of the projected light or sound is just detectable may be regarded as a "boundary" distance. Where this boundary distance is longer than a near point of the hyperfocal distance, the object falls within the depth of field regardless of the position of the object between a minimum and infinity. Therefore no problem arises.
For example, in a 35 mm camera having a focal length f=35 mm, F-number=2.8 at full open aperture, and a disc of least confusion .delta.=0.03, the near point of the hyperfocal distance is found to be H/2=f.sup.2 /2(.delta.f)=7.3 meters. If the boundary distance is taken at 10 meters, automatic focus adjustment is possible throughout the entire focusing range. However, for a video camera of f=60 mm, F/1.8 at full open aperture, and a disc of least confusion .delta.=0.03 mm, H/2 is found to be 33 meters. This means that it is very difficult to realize a camera having reliable focusing control throughout the entire focusing range given the available sorts and sizes of electrical elements.
Though a majority of the prior known active type range finders can be practically used in most photographic situations, focusing has to be performed manually in some situations where the diaphragm aperture is relatively near fully open, for example, a distant view of fireworks or neon signs at night. For example, in the aforesaid video camera having the lens of f=6 mm, assuming the boundary distance of light or sound is 10 meters, from 10,000=60.sup.2 /2(0.03F), we obtain F=6. The use of a diaphragm aperture opening larger than F6 will often result in defocused photographs.
Conversely, the passive type automatic focus adjusting device is, in principle, reliable only when the object to be photographed has some distinguishable contrast. For this reason, there is the drawback that the device is unable to measure the distance in situations where the object is a wall having no contrast, or the like, or where, because of low illumination, a difference in the contrast is indiscernible. However, as opposed to the active type, the passive type, when applied to objects of distinguishable contrast, is able to measure the distance accurately no matter how long the object distance may be. In conclusion, the active type range finder is more effective for the shorter half of the focusing range, while the passive type is more effective for the longer half. Moreover, these two types can be said to compensate for each other's drawbacks.
For such reasons, the employment of both the active and passive type range finders in a single camera is known. In the prior art, however, the hyperfocal distance and the boundary distance have not sufficiently been taken into account. As the diaphragm is relatively closed down, the photographic objective is stopped at the hyperfocal distance for that aperture size or the full open aperture. Thus, even in the in-focus condition, the passive type range finder continues operating and measuring the distance. As a result, the objective executes fine excursions so that an unpleasant photograph of a moving body is taken. Also, in panning photography, when a scene of little contrast appears in the field of view of the finder, the passive type range finder automatically switches to the so-called search mode, with the result that the objective is moved a large distance. Thus, in some cases, the use of the conventional dual-method dependent automatic focus adjusting device in shooting a moving body provides worse pictures.